Hybrid Vigor 173 



vigor, it would follow tlial the most vigorous plant will 

 be the one containing the greatest number of dominant 

 factors. It has been shown that the j)lants contain- 

 ing the greatest number of dominant factors are the 

 hybrids; it is for this reason that hybrids are relati\ely 

 vigorous. 



The following question may be raised. If it is granted that 

 most desirable factors tend somewhat to increase the general vigor, 

 do they all do this to the same degree ? The natural answer is in 

 the negative, but this has no bearing upon the validity of the 

 explanation. On the other hand, if heterozygosis be accepted for 

 an explanation the question presents a dilhculty. Heterozygosis 

 would suggest that Aa induces vigor, not because of any particular 

 factor that it represents, but because it is a heterozygous set. 

 It seems more reasonable and natural to suppose that certain 

 factors induce more vigor than others. 



It is evident that the suggestion made above is that 

 of a real explanation of hybrid vigor and not merely a 

 restatement. Keeble and Pellew (id) suggested it in 

 1 9 10, and since that time it has been somewhat discussed 

 in the literature, being referred to as ''the hypothesis 

 of dominance (accounting for hybrid vigor)." At lirst 

 statement the theory seems sound, but actually it does 

 not fit the facts. The two outstanding objections to 

 this theory of dominance are brought out in the })ublica- 

 tions of Shull, Emerson, and East. 



I. If hybrid vigor were due to dominance, it would be 

 possible in generations subsequent to the F, lo recom- 

 bine in one race all of the dominant factors in the homo- 

 zygous condition. Thus there could be isolated a race 

 that was ''100 ])er cent vigorous," and since il would 

 be homozygous, its vigor would not be lost by inbreeding. 

 Actually, though, no one has (as yet) been able to "lL\" 



